Consider the STOCK Act of 2012. It already makes insider trading illegal for members of Congress, with real penalties. So we already have a law targeting the actual harm—using nonpublic information—without the blunt instrument of a blanket ban. The problem with that law is weak enforcement and disclosure, not the absence of a trading ban.
A full ban treats every public official like a potential criminal. It also pushes talented people out of government, because they'd have to liquidate or blind-trust their entire portfolio. That's a real cost we don't talk about.
We don't ban doctors from owning healthcare stocks. We police their behavior. Same logic applies here. Officials aren't inherently corrupt; they're people with a job. Hold them to a clear standard, enforce the rules we've got, and make transparency mandatory. That solves the problem without stripping away a basic right to invest in your own country.